Gut-brain signal

Vagal gut–brain signaling — the nerve highway relaying gut and microbial state to the brain.

Gut barrier increases it.

Bonaz 2018, Front Neurosci 12:49 — vagal afferents sense gut luminal and microbial signals (via EEC-released 5-HT/CCK/GLP-1), relaying gut state to the brain. PMC5808284

Mood (serotonin) increases it.

The gut talks to the brain in serotonin: enterochromaffin cells synthesize about 95% of the body's 5-HT and release it in response to mechanical or chemical stimulation of the gut, whereupon it acts on 5-HT3 receptors on the terminals of vagal afferents to increase vagal afferent firing (PMC4625078, PMC5165017). The vagus is roughly 70-80% sensory, so this is predominantly gut reporting to brain rather than brain commanding gut — and 5-HT3 signaling on that route carries satiety and visceral sensation. PMC4625078

Innate immune sensor increases it.

TLR4 receptors ARE expressed on vagal afferent fibers themselves (Goehler 1999), so the nerve carries its own bacterial sensors rather than relying on relayed signals — and LPS activates vagal afferents directly at the level of the NODOSE ganglia (Hosoi 2005). The vagus is not the only route, and the source says so: subdiaphragmatic VAGOTOMY does not completely block the brain-mediated behavioral and neural effects of peripheral LPS or IL-1-beta (Schwartz 1997) — humoral routes persist. doi 10.3389/fnins.2018.00049 · PMC12293201

Appetite & sugar hormone increases it.

10.5009/gnl.2012.6.1.10 — GLP-1 and PYY act on the gut-brain axis by binding receptors in vagal afferents, as well as crossing the blood-brain barrier to reach CNS receptors and regulate POMC/CART and AgRP/NPY neurons directly — two parallel routes, neural and humoral. Vagal afferents terminate in the nucleus of the solitary tract, which relays to locus coeruleus, dorsal raphe and hypothalamus. doi 10.5009/gnl.2012.6.1.10

Pathogen tripwire increases it.

And the random test turned up a necessity demonstration that bears directly on probiotic claims: after VAGOTOMY, the beneficial effects of Lactobacillus and Bifidobacterium on cognitive function ARE abolished, and the positive effects of probiotic ingestion on cognition ARE only evident when the vagus nerve is intact. The nerve is not one route among several for this effect — it is required. shown by lesion in animals; the vagus is stated to be the primary sensory pathway for visceral information reaching the CNS despite there being NO direct contact between gut microbiota and vagal afferents — sensing is via diffusing metabolites and enteroendocrine intermediaries. SCFA specificity: butyrate acts directly on afferent terminals while oleate acts through a CCK-mediated mechanism — different metabolites, different routes. PMC12293201

Sleep increases it.

Sleep deprivation reduces the release of vagal neurotransmitter acetylcholine and REM sleep deprivation in rats led to significant deterioration in cardiac autonomic regulation. Species: the acetylcholine and REM-deprivation arms are rodent; the human arm is heart-rate-variability meta-analysis, which is association. doi 10.1016/j.smrv.2011.05.001 · PMC3132857

Calms body-wide inflammation increases it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Immune support increases it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Lowers cortisol increases it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Cardiac output increases it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Sympathetic drive reduces it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Memory chemical increases it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Stress & cortisol reduces it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

Women's hormone balance increases it.

Every link on this pathway inherits one taVNS produced opposite effects split by baseline RMSSD. High baseline tone - heart rate and TNF-alpha significantly increased and vagally mediated HRV significantly decreased versus sham. The same intervention helped one group and harmed the other. Diagnostic stratification failed where biological stratification worked: NO difference in any outcome between MDD participants and controls. doi 10.3390/biomedicines13030700 · PMC12689627

It increases Fullness hormone (CCK).

Bitter taste receptors (T2Rs) comprise a family of 25 G protein-coupled receptors that are expressed in multiple tissues, including enteroendocrine cells (EECs), of the GI tract. 10.1093/ajcn/nqab418. doi 10.1093/ajcn/nqab418

It reduces Inflammatory alarm.

The founding experiment is a cut and a current: transection of the vagus in rats subjected to LPS-induced endotoxaemia led to a more aggressive systemic inflammatory response, characterized by earlier onset of shock and higher serum and liver TNF-alpha. And electrical stimulation of the distal arm of the transected vagus attenuated this response. Stimulating the cut end proves the signal runs outward — the brain is not merely being informed of inflammation, it is switching it off. And the receptor is proven necessary by genetics, not inferred: electrical stimulation significantly attenuated endotoxin-induced serum TNF in wild-type mice. Vagus nerve stimulation in alpha-7 nicotinic acetylcholine receptor deficient mice, however, failed to reduce serum TNF. Same stimulation, same endotoxin, one missing subunit, no effect. The cellular route is stranger than a nerve-to-macrophage synapse and worth knowing: the acetylcholine is produced by choline-acetyltransferase-positive T cells, which act on macrophages expressing alpha-7 receptors. A nerve talks to a lymphocyte and the lymphocyte makes the neurotransmitter. And SPLENECTOMY inactivates the pathway — the spleen is the effector organ. Why this arrow points at the inflammasome specifically: alpha-7 nicotinic acetylcholine receptor signaling inhibits inflammasome activation by preventing mitochondrial DNA release. That is the precise molecular target, and the NLRP3 inflammasome already reaches neuroinflammation and atherosclerosis on the map — so this link connects the gut-brain axis to the CHIP/atherosclerosis mechanism through a shared node neither review was looking for. It reaches clinic: vagus nerve stimulation inhibits cytokine production and attenuates disease severity in rheumatoid arthritis, with trials extending to lupus, gout and inflammatory bowel disease. the mechanistic work is rodent; the clinical work is open-label or small-trial. The Borovikova and Wang findings are cited from the primary literature. doi 10.1073/pnas.1605635113 · PMC9381415

It increases Cardiac output.

The right vagus predominantly innervates the sinoatrial node, modulating heart rate, while the left vagus more strongly influences atrioventricular conduction. Anatomically specified and lateralised. And it is directly demonstrable in a healthy person: the mammalian diving reflex occurs when sensory fibers of the vagus and trigeminal nerve ARE stimulated by facial SUBMERSION to trigger bradycardia. Respiratory sinus arrhythmia — heart rate accelerating on inhalation and decelerating on exhalation — is driven by vagal modulation, and high resting vagal tone, indexed by high-frequency HRV, is associated with lower resting heart rate and more efficient baroreflexes. This synchronisation is a critical predictor of survival in coronary heart disease — which is why HRV is a prognostic measure and not merely a wellness metric. PMC12082064

It reduces Brain inflammation.

The vagus nerve plays a role in modulating inflammation and immune responses through the cholinergic anti-inflammatory pathway, and treatments that target the vagus nerve increase the vagal tone and inhibit cytokine production. It is described as A NEUROIMMUNE feedback loop rather than a one-way output, which matters because neuroinflammation is also on the inbound list for this pathway — cytokine load reduces vagal tone. Clinical arm: vagus nerve stimulation is a promising ADD-ON treatment for treatment-refractory depression, posttraumatic stress disorder and inflammatory bowel disease. The reviewers call that evidence preliminary and the word is theirs. doi 10.1016/j.bbi.2024.11.009 · PMC5859128

It increases Mood.

Low vagal tone has been linked to an increased risk of depression, associated with reduced parasympathetic activity and increased sympathetic dominance. The mechanism is anatomical, not correlational: the stimulation of vagal afferent fibers in the gut influences monoaminergic brain systems in the brain stem that play crucial roles in major psychiatric conditions, such as mood and anxiety disorders. That is a named projection to a named target. The reviewers describe the evidence as preliminary. doi 10.3389/fpsyt.2018.00044 · PMC5859128

It reduces Sympathetic drive.

HRV captures the dynamic balance between sympathetic and parasympathetic inputs to the heart, and a low vagal tone, as assessed by heart rate variability, is a marker of the sympatho-vagal balance. Low vagal tone is characterized as reduced parasympathetic activity and increased sympathetic dominance. The measure and the mechanism ARE entangled here and it is worth saying: HRV is used both as the index of vagal tone and as the evidence for sympatho-vagal balance, so this edge risks being partly definitional. Drawn because the reciprocal innervation of the sinoatrial node is independently established anatomy, not because the ratio moves. PMC12082064

It increases Digestion.

The enteric nervous system and vagal signaling govern motility. doi 10.1038/s41522-022-00352-6 · PMC11265038

It reduces Calms body-wide inflammation.

The drivers mirror neuroinflammation’s where the same evidence supports both, and the inflammaging reviews name them jointly: nutritional perception dysregulation and inflammatory aging ARE the core key elements linking upstream molecular damage to downstream organ function decline. Where the driver is shared with neuroinflammation the evidence is the same evidence - the split was about what these nodes drive, not about what drives them, and that asymmetry is stated rather than hidden. doi 10.1016/j.molmet.2023.101755 · PMC12293478

It increases Stress cascade trigger.

Drawn from the reviews of this outcome.

It increases Gut barrier.

10.3389/fnins.2026.1878765 — The aging outcome lists the thirteen hallmarks themselves, each mapped to its pathway and labeled with its hallmark number. (from the reviews of this outcome). Verbatim: “Brief non-invasive vagal nerve stimulation consistently reduces paracellular permeability of the small intestine after CRH administration, but does not entirely mitigate I-FABP release from the epithelium”. doi 10.3389/fnins.2026.1878765 · PMC9786250

It increases Curbs appetite.

10.1038/s41598-025-31377-4 — The aging outcome lists the thirteen hallmarks themselves, each mapped to its pathway and labeled with its hallmark number. (from the reviews of this outcome). Verbatim: “DCA-induced c-Fos expression was eliminated following truncal vagotomy or silencing of TGR5 in the NG”. doi 10.1038/s41598-025-31377-4 · PMC7453905

It increases Stress resilience.

10.3389/fpsyg.2025.1639866 — The aging outcome lists the thirteen hallmarks themselves, each mapped to its pathway and labeled with its hallmark number. (from the reviews of this outcome). doi 10.3389/fpsyg.2025.1639866

It increases Calm pain signaling.

The defining feature is that the pain is real and the tissue is not damaged: central sensitization manifests as hyperalgesia, allodynia, expanded receptive fields, and impaired endogenous pain inhibition, and in osteoarthritis the same phenomenon does not correlate with radiographic findings - severe pain may persist despite stable joint structure. The mechanism is a balance failure in both directions at once: in the dorsal horn there is excessive release of glutamate, ATP, and substance P, leading to activation of NMDA receptors - while at the same time descending inhibitory mechanisms, notably those mediated by GABA, ARE weakened. Too much excitation and too little inhibition. And mast cells carry a specific hypothesis: thalamic mast cells releasing histamine, IL-1-beta, IL-6, TNF, CGRP and substance P, either stimulating thalamic nociceptive neurons directly or via microglia. Stated by the authors as a hypothesis, not an established mechanism. The reviewers ARE blunt about the state of knowledge: the pathophysiology and etiopathogenesis remain largely unknown, and there is NO definitive pathogenesis except for altered central pain pathways. doi 10.3390/biomedicines12071543 · PMC6687840

It increases Stomach contents rising.

Drawn from reviews of neighboring outcomes rather than from studies of this outcome itself, which is weaker evidence, and the map marks it so. Sources: doi 10.1038/s41522-022-00352-6; PMC5559637. doi 10.1038/s41522-022-00352-6 · PMC5559637

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